Water clarity - Understanding Lake Data

PHYSICAL CHARACTERISTICS - Page 6

Water clarity

Strictly speaking, clarity is not a chemical property of lake water. More accurately, it is an indicator or measure of water quality related to chemical and physical properties.

Water clarity has two main components: true color (materials dissolved in the water) and turbidity (materials suspended in the water such as algae and silt). The algae population is usually the largest and most variable component.

Water clarity often indicates a lake's overall water quality, especially the amount of algae present. Algae are natural and essential, but too much of the wrong kind can cause problems. Table 2 shows the inverse relationship between Secchi disc depth (a measure of clarity) and chlorophyll a (a measure of algae) for different types of lakes.

Secchi disc (Secchi disk) readings are taken using an 8-inch diameter weighted disc painted black and white. The disc is lowered over the downwind, shaded side of the boat until it just disappears from sight, then raised until it is just visible. The average of the two depths is recorded. Secchi disc readings should be taken on calm, sunny days between 10 a.m. and 2 p.m. since cloud cover, waves, and the sun's angle can affect the reading.

Table 2. Water clarity index.

Water clarity	Secchi depth (ft.)
Very poor           3
Poor                5
Fair                7
Good               10
Very good          20
Excellent          32

Page - 7

Secchi disc values vary throughout the summer as algal populations increase and decrease. Measuring several sites may be useful in some lakes, depending upon the uniformity of the lake. Year to year changes result from weather and nutrient accumulation. Weekly or biweekly Secchi records (April-November) over a number of years provide an excellent and inexpensive way to document long-term changes in water clarity.

The color of lake water reflects the type and amount of dissolved organic chemicals it contains. Measured and reported as standard color units on filtered samples, color's main significance is aesthetic. Color may also reduce light penetration, slowing weed and algae growth. Many lakes possess natural, tan-colored compounds (mainly humic and tannic acids) from decomposing plant material in the watershed. Brown water can result from bogs draining into a lake. Before or during decomposition, algae may impart a green, brown or even reddish color to the water.

Color can affect the Secchi disc reading. Table 3 lists color values associated with varying degrees of water color.

TABLE 3. Water color. (Adapted from Lillie and Mason, 1983.)

	0-40 units	Low
      40-100 units	Medium
	>100 units	High

Another measure of water clarity, turbidity is caused by particles of matter rather than dissolved organic compounds. Suspended particles dissipate light, which affects the depth at which plants can grow.

Turbidity affects the aesthetic quality of water. Lakes receiving runoff from silt or clay soils often possess high turbidities. These values vary widely with the nature of the seasonal runoff.

Suspended plants and animals also produce turbidity. Many small organisms have a greater effect than a few large ones. Turbidity caused by algae is the most common reason for low Secchi disc readings.

The Wisconsin Department of Natural Resources (DNR) operates a "Self-Help Monitoring Program" for lakes. Local volunteers take Secchi disc and other readings and the DNR provides computer data storage and annual reports. For more information, contact a district DNR office or write to:
DNR Lake Partnership FH/3
P.O. Box 7921
Madison, WI 53707-7921.

Back to Cover/Index...Page1/2 , Next

For more information on this topic, contact:

James Vennie
Watershed Management
(608) 266-2212

Lakes Partnership | Watershed Management | Fish Wisconsin | Fisheries Management

Last Revised: Tuesday February 05 2008